Characterization of apatite within the Mactung W (Cu,Au) skarn deposit, Northwest Territories : implication for the evolution of skarn fluids

Abstract

Skarn-hosted apatite from the Mactung deposit, Selwyn Basin, Northwest Territories, Canada, is characterized to understand the chemical evolution and signatures of mineralizing fluids. Petrography and rare earth element (REE) abundances constrain three generations of fluorapatite, each of which record distinct moments in the system’s evolution. Type-i apatite occur with detrital phosphate nodules. This apatite contains variable ΣREE+Y (1314 ± 821 ppm, 1σ, n = 78) and show negatively sloping chondrite normalized REE+Y (REE+Y_N) patterns with variable LREE_N/HREE_N (La_N/Lu_N = 27 ± 30) and weak Eu anomalies (Eu_N/Eu_N* = 0.6 ± 0.2; Eu* = √Sm_N*Gd_N). The REE abundances of phosphate nodules from Mactung are similar to those reported in unmineralized units at Howards Pass in the Selwyn Basin. These REE patterns are similar to that of type-i apatite, suggesting that type-i apatite are recrystallization products of phosphate nodules, likely during isochemical contact metamorphism. Hydrothermal type-ii apatite occur with skarn assemblages and are divided into two sub-types: i) early type-ii apatite occur in prograde skarns and show high flat lying REE+Y_N patterns (e.g., ΣREE+Y = 17194 ppm, La_N/Lu_N = 3.5, n = 31 ) with negative Eu anomalies (e.g., Eu_N/Eu_N* = 0.1) and ii) later type-ii apatite occur in retrograde skarns and show bowl shaped to negatively sloped REE+Y_N patterns, reflecting low MREE content (e.g. La_N/Sm_N = 10.2, Sm_N/Lu_N = 0.3, n = 17), and positive Eu anomalies (e.g., Eu_N/Eu_N* = 14.3). Type-iii apatite are associated with quartz-scheelite veins, which cross-cut prograde skarn. These apatite contain high ΣREE+Y (7752 ± 496 ppm, n = 3) and exhibit bowl-shaped REE+Y_N patterns, corresponding to low MREE (La_N/Sm_N = 8 ± 0.3), and no Eu anomaly. The REE patterns and paragenesis of hydrothermal apatite (i.e. type-ii and type-iii) indicate the presence of two chemically distinct ore fluids during the different stages of mineralization of the Mactung deposit. The first fluid is associated with prograde skarns and early type-ii apatite, whereas the second fluid, recorded in late type-ii and type-iii apatite, formed retrograde skarn and quartz veins. The REE+Y_N patterns of late type-ii and type-iii apatite record a MREE fractionation through early crystallization of MREE-rich minerals.